New working methods in the mining of hard coal and the use of ever larger and more powerful machines in obtaining the coal has resulted in an increase in the finest grain of .ltoreq.0.5 mm in the feed for hard coal processing. This set of problems occurs also in the processing of waste tips, which has recently gained considerably in importance. This proportion of finest grain can be far above 20%. Especially problematical and of great consequence for the processing of this finest grain is the increase in the ultra-fine grain of .ltoreq.5 .mu.m which, in turn, may constitute up to 30% of the finest grain. This ultra-fine grain has very high ash contents and consists predominantly of inert mineral components.
The almost colloidal nature of this ultra-fine mineral material hampers the grading of the finest coal which is usually carried out with the acid of flotation. This is reflected in higher consumption of reagents and long residence times in the flotation machines and also in higher ash contents in the flotation concentrate.
The presence of this ultra-fine mineral material poses a considerably greater problem in the dewatering of the flotation concentrate. Owing to the large surface area of the ultra-fine mineral material and also the mineralogical structure, a very great deal of water is retained, as a result of which, especially, the water content of the flotation concentrate is unfavourably influenced. In addition, the mineral material causes great difficulties as a result of blocking the capillaries of the filter cake and the openings in the filter cloth, as a result of which the residual water content of the concentrate frequently cannot be brought below 20%.
Attempts to remove the ultra-fine mineral material either from the raw raised coal or from the finest coal (flotation feed) have hitherto met with little success. Attempts to carry out separation by mechanical means, such as centrifuges or cyclones or by washing or hydrosizing in a gravitational field have probably been frustrated by the fine particles agglomerating, as a result of gravitation or surface-attraction forces, on the coarser particles present in a pulp. This bond is very strong and cannot be broken by mechanical action, for example attrition. Separation methods based on, or decisively influenced by, electrochemical processes are faced with the difficulty that the ultra-fine particles of the finest-grained mineral material which contaminates the coal possess, in the neutral pH range in which coal processing is usually carried out, a high negative surface potential whereas the coal itself has a positive or only a slightly negative potential. This difference in potential between mineral material particles and coal particles results in coagulation of the coal particles with the mineral material particles.
A possible solution to the problem here could be selective flocculation. The object of selective flocculation is to flocculate one component in a pulp containing several mineral components and allow it to sediment whilst the other components remain dispersed. In ore processing and in the processing of mineral slurries processes have also already been proposed for bringing about, with the aid of selectively acting flocculating agents both of inorganic and organic origin, a selective flocculation of the individual components of a pulp (Muller et al., Erzmetall 33, 1980, pages 94 to 99). It has not been possible, however, especially in the case of high molecular flocculating agents based on polyelectrolytes, to achieve a high selectivity such that one component is flocculated as quantitatively as possible and can therefore be separated, since an unselective adsorption virtually always occurs owing to non-ionogenic groups and the effect of potential-determining ions in the mineral pulp.